Even in regions of the world that are not geologically active, the vertical temperature gradient in the earth crust usually exceeds 20° C./km. Hence at a depth of 5000 m, rock temperatures exceeding 100° C. are expected. There are large geographical variations. In some areas not considered geologically active, thermal gradients in the crust exceed the indicated number by a factor of two or more. Gradients lower than 20° C./km appear to be uncommon. High gradients are typically experienced where insulating layers of rock are stacked on top of heat producing rock or intrusions of eruptive rock. Hot Dry Rock (HDR) is rock formations with low porosity and with no natural aquifers. In such rocks heat transfer takes place mainly through conduction. Given the very low conductivity of most types of rock, heat transfer must be expected to be low in the rock. Therefore, efficient methods must be devised in order to extract heat from the rock.
Several methods for exploiting the significant thermal energy stored in HDR formations have been proposed and tested. The most common method consists of drilling one or more water injection holes and a production hole at a different location. By fracturing the rock between the injection and production holes, a closed circuit for water flow can be established in the rock. While drilling costs can be lower than for many alternatives, there are several practical challenges related to the proposed methods. First, it is difficult to control the fracturing process and thereby the establishment of the heat exchanger in the geological formation. Second, water flow in the heat exchanger is difficult to control and optimize as the water will tend to follow the path of least hydraulic resistance and not the path for optimal heat production. Thirdly, the fractured volume is difficult to maintain as the fractures may get clogged and cannot easily be re-opened. Research and development of fractured rock designs has been pursued for half a century with limited practical success.
Recently, methods for extraction of energy from HDR formations depending heavily on drilling technology have been proposed. US 2007/0245729 A1, DE 10 2005 036 472, EP 1 995 457, US 2007/02457729 A1, US 2011/0048005 A1 and US 2011/0061382 A1 describe energy plants applicable for HDR, essentially producing from a set of separate wells with supply and return holes widely separated and connected by single horizontal production holes. A large number of wells are needed to allow a reasonable large heat output, and supply and return holes widely separated would give a rather impractical plant design. In U.S. Pat. No. 5,515,679, U.S. Pat. No. 7,251,938, and U.S. Pat. No. 6,247,313 the hot liquid is returned through a common return hole, and injection and return holes need not be separated. US2011/0067399 A1, CA2679905 A1, DE 43 19 111 A1, US2008/0169084 A1 and WO 2010/021618 A1 describe energy plants applicable for HDR consisting of single wells with integrated supply, production and return holes. The wells consist of single a hole with an internal pipe separating water flow downward and upward water flow. The water is injected in the outer annulus and is gradually heated until reaching the lower end of the pipe where it returns in the inner pipe or vice versa. An alternative version of such a well is given in U.S. Pat. No. 6,000,471 where water injection and return takes place through separate holes, a more expensive alternative. The challenge for all such solutions is to ensure sufficient heat transfer from the rock. A large number of such wells would be required in order to obtain a satisfactory amount of energy output. With significant cost of drilling it would be impractical to establish multiple injection and return wells. Alternatively the method must be combined with fracturing of the rock and to ensure circulation of fluid in the rock. As discussed above, U.S. Pat. No. 6,247,313 describes a geothermal energy plant for HDR formations consisting of multiple production holes and at least one supply and one return hole. The energy plant offers the solution to many of the challenges related to the previously presented inventions. Heat is efficiently being extracted from the rock formation by optimized spacing of production holes. Drilling costs are kept reasonably low by reducing the number of supply and return holes. The concept assumes at least one supply and one return hole. Furthermore, the production holes are assumed to be inclined between 20 and 50 degrees relative to the vertical axis.
Whereas HDR can most efficiently be established through drilling of a set of multiple production holes through the rock formation, it is important to minimize drilling costs. Typically drilling cost constitutes 90% of the cost of establishing the geothermal plant. Supply and return holes are particularly expensive to drill and do not contribute much to heat production. Hence their length should be minimized.